1. Field of the Invention
Generally, this invention relates to a device for monitoring deformation of natural or man-made slopes, supporting structures of ground excavation, or of long structures such as the pipeline for transferring oil, gases or water, or beams and columns of buildings, bridges or ships; wherein, the deformation is monitored by means of an automated displacement monitoring probe with high sensitivity, durability and stability. To achieve optimum performance, the diameter of the monitoring probe and its distributed density subject to the characteristics of the target can be adjusted.
2. Description of the Prior Art
According to Green, G. E., and Mickkelsen, P. E., “Deformation Measurements with Inclinometers”, Transportation Research Record 1169, TRB, National Research Council, Washington, D.C. (1988), S. D. Wilson of Harvard University developed the concept of a probe inclinometer system in 1952. Today, the inclinometer system is probably the most widely used technique in the detection of ground movements. An inclinometer casing made of plastic or aluminum is installed in a near vertical position in the ground. For monitoring the stability of an earth slope, an inclinometer probe (IP) equipped with wheels that fit tightly with the grooves in the inclinometer casing is typically used to serve as the sensor unit. An electric cable raises and lowers the IP in the casing and transmits electric signals to the ground surface. The IP measures the inclination of the inclinometer casing in reference to verticality. Readings from the sensor unit are taken typically at a fixed interval of 500 mm as the probe is raised or lowered in the casing. The displacement at any depth of the casing is determined according to the IP inclination measurements. The aforementioned method is usually carried out manually which is time consuming. An in-place-inclinometer (IPI) probe is available that places the sensor probes in the ground on a long term basis and allows automated data logging. The above-described ground displacement monitoring devices use an electrical system for sensing and signal transmission. The electrical systems are prone to short circuit when exposed in a humid environment such as underground and below ground water. Most of the electrical sensors are non-distributive in nature where one transmission line is dedicated to a specific sensor. When a large number of sensors are used, the equally large number of transmission lines can make the system impractical. The electrical signals are subject to electromagnetic interference. These drawbacks make the electrical ground movement monitoring systems complicated or expensive to use.
The pipe strain gauge, which has been disclosed in Annual Report of Study for Hazard Prevention in Tokyo, the papers of Takada, Y.; Kyodai B. and Kenkyu N., “Measurement of international strain on landslide occurring ground” No. 8, P. 586 (1965) and Nakamura, H.; Landslides, “A study of finding landslide surface by the use of buried strain meters” Vol. 6, No. 1, pp. 1˜8 (1969), respectively, use the principle of flexural strain caused by bending of a flexible pipe. The pipe strain gauge consists of a series of strain gauges attached to the surface of a flexible pipe. By sensing the flexural strain, the deformation distribution perpendicular to the longitudinal axis (as shown in FIGS. 1 (a) and 1 (b)) can be monitored. The pipe strain gauge may be used to determine the direction of ground movement 01 and location of the sliding surface 02 in FIGS. 1 (a) and 1 (b). The strain gauge is a non-distributive, electric sensor and thus shares similar drawbacks as the IPI.
Shiang et al., “Optical Fiber Sensing Technology, and Introduction of Implanted optical Fiber Bend Meter and Test Application”, Proceedings of 12th Non-destructive Detection Technology Symposium (2004), pp. 273˜279, described a device (called the optical fiber bend meter (as shown in FIG. 1 (c)) that uses a pair of stretched fiber Bragg gratings (FBG) 03 to measure ground displacement in combination with the inclinometer casing. The design of the bend meter was based on the concept reported by Yoshida, Y., Kashiwai, Y., Murakami, E., Ishida, S., and Hashiguchi, N., 2002, “Development of the monitoring system for slope deformations”, Proceedings, SPIE Vol. 4694, pp. 296˜302. The bearing 04 shown in FIG. 1 (c) allows rotation of the rigid column 05. The bend meter can be inserted inside an inclinometer casing. The ground movement causes bending of the inclinometer casing and that bending forces rotation of the rigid column 05 and simultaneous extension/compression of the two fiber Bragg gratings (FBG) 03. The amount of relative rotation or rotation of the bottom piece around the hinge 04, is determined by the differential elongation between the two pre-stressed optic fibers inscribed with FBG's. Because of the flexible nature of the optic fiber, the measurement mechanism is effective only if the optic fibers remain tensioned. For this reason, the FBG optic fibers in the bend meter are pre-stressed. Furthermore, the extension/compression sensed by the FBG's are not resulted only from the deflection of the bend meter. All longitudinal forces including the weight of the bend meter units and friction between the bend meter support and the grooves in the inclinometer casing can all affect the readings for such design of FIG. 1 (c). It is possible therefore, that non-repeatable and/or unpredictable errors can occur while using the bend meter to monitor ground displacement.
Though there have been many types of optic fiber sensors available commercially, these sensors are not always dedicated for ground displacement monitoring. They lack the necessary sensitivity and/or compatibility with the currently available ground displacement monitoring systems.